An anechoic chamber is a room in which acoustically free field conditions exist. For practical measurements, it must also be clear of extraneous noise interferences. An environment meeting these conditions is a requirement for precision acoustical measurements. Anechoic chambers are widely used in the development of quieter products in many industries and institutions including the following: aircraft, electrical, transportation, communications, business machines, medical research and universities.
An acoustical free field exists in a homogenous, isotropic medium which is free from reflecting boundaries. In an ideal free field environment, the inverse square law would function perfectly. This means that the sound pressure level (L.sub.p) generated by a spherically radiating sound source decreases six decibels (6 dB) for each doubling of the distance from the source. A room or enclosure designed and constructed to provide such an environment is called an anechoic chamber.
An anechoic chamber usually must also provide an environment with controlled sound pressure level (L.sub.p) free from excessive variations in temperature, pressure and humidity. Outdoors, local variations in these conditions, as well as wind and reflections from the ground, can significantly and unpredictably disturb the uniform radiation of sound waves. This means that a true acoustical free field is only likely to be encountered inside an anechoic chamber.
For an ideal free field to exist with perfect inverse square law characteristics, the boundaries must have a sound absorption coefficient of unity at all angles of incidence.
Conventionally, an anechoic element is defined as one which should not have less than a 0.99 normal incidence sound absorption coefficient throughout the frequency range of interest. In such a case, the lowest frequency in a continuous decreasing frequency sweep at which the sound absorption coefficient is 0.99 at normal incidence is defined as the cut-off frequency. Thus, in an anechoic chamber, 99% of the sound energy at or above the cut-off frequency is absorbed. For less than ideal conditions, different absorption coefficients may be established to define a cut-off frequency.
As mentioned above, another characteristic of a true free field is that sound behaves in accordance with the inverse square law. In the past, testing wedges in an impedance tube has been a means for qualifying wedges used in chambers simulating free field conditions. A fully anechoic room can also be defined as one whose deviations fall within a maximum of about 1-1.5 dB from the inverse square law characteristics, depending on frequency. Semi-anechoic rooms, i.e., rooms with anechoic walls and ceilings which are erected on existing acoustically reflective floors such as concrete, asphalt, steel or other surfaces, can deviate from the inverse square law by a maximum of about 3 dB depending on frequency.
The table below reflects the maximum allowable differences between the measured and theoretical levels for fully anechoic and semi-anechoic rooms:
______________________________________ Maximum Allowable Differences Between the Measured and Theoretical Levels One-Third Octave Band Centre Allowable Type of Frequency Differences Test Room Hz dB ______________________________________ Anechoic &lt;630 .+-.1.5 800 to 5,000 .+-.1.0 &gt;6,300 .+-.1.5 Semi-anechoic &lt;630 .+-.2.5 800 to 6,000 .+-.2.0 &gt;6,300 .+-.3.0 ______________________________________
Because of the very high degree of sound absorption required in an anechoic chamber, conventional anechoic elements typically comprise fully exposed sound absorptive material or sound absorptive fill elements which are covered with a wire cage to contain and somewhat protect the sound absorbing material. Typical wire mesh coverings have approximately 90-95% open space to allow maximum exposure of sound absorbing material to the sound waves, yet providing a certain level of protection for the material.
A disadvantage with anechoic construction elements as explained above is that in highly industrial environments the wire mesh structure may not provide sufficient physical protection for the elements. The sound absorbing material can therefore become easily disfigured by unintentional impact that is quite foreseeable in a heavily industrial environment.
Another disadvantage of the conventional anechoic elements is potential medical hazards. The sound absorptive materials such as fiberglass, rockwool or foams can be highly erosive. Over a period of use such materials could erode into particulate matter floating in the air which could be inhaled into lungs.
A further disadvantage of the conventional anechoic elements and their wire mesh coverings is that in highly industrial applications, oil spills and dirt may rapidly accumulate on the sound absorbing materials. This may impede sound absorption performance of the material and additionally may impose a fire hazard. Cleaning the sound absorptive material is difficult and not efficient.
Therefore there is a need for an anechoic element which provides a very high degree of sound absorption capabilities and sufficient protection for the sound absorbing material.